An article in today's MacWorld tells of a similar problem recently faced by another presenter: Steve Jobs, CEO of Apple. While demonstrating the new iPhone 4, the wi-fi connection failed. That was a critical part of Jobs' demonstration. Author Thomas Wailgum first describes how Steve Jobs handled the situation, then goes on to offer hints of how you can handle unexpected problems when you are making presentations.
If you ever make presentations to any group, I'd suggest you read Thomas Wailgum's article right now at http://www.macworld.com/article/151903/2010/06/stevejobs_presentations.html?lsrc=nl_mwweek_h_cbstories and then follow his advice BEFORE your next presentation.